Gas separation membranes can potentially be used in applications such as steam reforming, water-gas shift reaction and dehydrogenation of hydrocarbons. Typically these processes are operated at temperatures above 200° C. Some of these processes are typically carried out at pressures above 10 bar or even above 30 bar. For organic polymers, increased macromolecular dynamics at such elevated temperatures and/or pressures, manifested by swelling and/or plasticization, diminish membrane perm-selectivity [see e.g. Koros 2001]. Even high glass transition polymers such as polyimides and polyaramides show a sharp decrease in selectivity at temperatures above 200° C. [Koros 2001]. Polyimides are often crosslinked in order to reduce chain mobility and CO2 plasticisation under demanding conditions (e.g. high pressure).
Ceramic membranes, such as amorphous silica, do not suffer from high chain mobility due to the rigidity of the silica network and show excellent gas separation properties at elevated temperatures based on molecular size exclusion [De Vos 1998]. However, difficulties in large scale processing of defect-free ceramic thin film membranes hinder application of such purely ceramic membranes. Ideally, gas separation membranes for applications at elevated temperature conditions should exhibit high permselectivity, stable selectivity and large-scale defect-free processability.
Verker et al. 2009, describe 25-30 μm thin films based on composites of polyimides and polyhedral oligomeric silsequioxanes (POSS). In these films, the POSS are distributed randomly throughout the polymer network. On a molecular scale the POSS are not distributed homogeneously; regions exist with and without POSS molecules. The composite is devised for withstanding hypervelocity impacts.
Reaction of aminated silsesquioxanes with trimesoyl chlorides by interfacial polymerisation (water/hexane) is reported to produce ultrathin (100 nm) films supported by an organic polymeric carrier material, after a reaction time of at least 5 minutes [Dalwani 2012]. The permeance for various small molecules in the liquid phase was studied at room temperature. No selectivity in the more sensitive applications of gas separation was suggested. The membrane formation was said to be easily extendible to other organic reactants.
Interfacial polymerisation as such (of polyamines with poly(acid chlorides)) was known in the art [Chern 1991; Chern 1992].
Pervaporation properties of polyimide membranes, including a membrane based on siloxane diamine and 6FDA (hexafluoro-isopropylidene-bis(phthalic anhydride)) have been reviewed [Jiang 2009].
Imides based on octa(aminophenyl)-POSS (OAPS) and pyromellitic dianhydride can be used for producing thick (0.50 mm) nanocompositie films having oxygen barrier functions [Asuncion 2007]. The gas permeability is thus expected to be extremely low for this type of films, which rules them out for the use of selective gas transport in membrane separation. The imides are produced in a homogeneous (N-methyl-pyrrolidone) medium. Mixing OAPS into polyimide based on a fluorinated dianhydride (6FDA) and m-phenylenediamine (MDA) affects the gas transport properties of the polymide membrane [Iyer 2010]. The POSS units of these mixed membranes are not an intrinsic part of the polyimide molecules. Nanocomposite membranes carrying mono-valent POSS units at the terminal positions of a polyimide also affect gas transport properties of the resulting thick (0.1 mm) membranes [Dasgupta 2010]. None of these prior art polymer membranes are alternating copolymers having the POSS units as repeating parts thereof, allowing to obtain ultrathin structures having effective gas separating properties under severe conditions (high temperatures and/or pressures).
There is a need for economically viable membranes which retain acceptable gas permeabilities and gas separation selectivities at more extreme operation conditions such as elevated temperatures and/or pressures, while avoiding deterioration of the performance of the membranes as a result of swelling or softening. The membranes as described above do not fulfil these requirements. Hence, an objective of the present invention is to develop gas separation membranes which exhibit these properties.